Metal materials having hydrophilic surface layers on their surfaces have been efficiently used in the whole industrial fields, which will now be explained by using a heat exchanger as an example.
A heat exchanger exchanging heat by making two fluids having different temperatures directly or indirectly contact each other has been widely used in various industrial fields, especially for heating, air conditioning, power generation, waste heat recovery and chemical processes.
An air conditioning heat exchanger forms a fin that is an enlarged surface at the air side to improve heat transmission. When the air having humidity passes through the fin in the heat exchange operation, heat transmission occurs by low temperature refrigerants supplied into a tube. When a temperature of the surface of the fin is equal to or lower than a dew point temperature of the air having humidity, droplets are formed on the surface of the heat exchanger to intercept flow of the air, which increases a pressure drop that is a pressure difference between the inlet and outlet of the heat exchanger. Accordingly, power of a fan must be increased to supply the same flux, which results in large power consumption.
In order to solve the above problem, as disclosed in Japanese Laid-Open Patent Application 61-8598, flow of condensed water formed on a surface of a fin of a heat exchanger is improved by performing an anti-corrosive process on an aluminum sheet of the fin by using Cr+6 to improve corrosion resistance, and performing a silicate-group coating process thereon to give hydrophilicity, which is called a pre-coated material (PCM).
The PCM basically requires Cr+6 to obtain corrosion resistance. However, Cr+6 will be prevented since 2006 due to environmental problems. There are thus increasing demands for a material substituting for Cr+6. Until now, Cr+3 or resin type has been suggested. In the preparation of the PCM, tetrachloroethane (TCE) inevitably used to wash aluminum also causes environmental contamination. In addition, the PCM showing excellent hydrophilic performance at an initial stage gradually loses the hydrophilic property, namely, has an aging characteristic. Recently, chemical products have been mostly used as materials for wallpaper. The silicate material for giving hydrophilicity is volatilized and chemically coupled with the wall paper, thereby discoloring the wall paper. Also, the volatilized materials displease people.
Continuous attempts have been made to satisfy various demands by forming a functional surface layer (for example, hydrophilicity or hydrophobicity) on the existing material. Exemplary methods for forming the functional surface layer include 1) depositing the functional surface layer on the existing material, and 2) giving new physical and chemical properties by improving the surface film of the existing material.
However, in the latter, as the time elapses, the surface property is changed and returns to the original property. For example, in the case that a metal such as aluminum is processed according to an ion beam assisted reaction process, hydrophilicity of the surface of the metal is improved. It is because a natural oxide film is etched on the surface of the aluminum and the functional film is formed thereon. As the time elapses, an oxide film is naturally grown on the surface of the aluminum. As a result, hydrophilicity improvement effects obtained by etching the natural oxide film are deteriorated. The functional film formed on the surface of the aluminum is made of an extremely thin layer (<a few nm) having very low mechanical resistance to environmental variations (water, temperature, etc.) by time. Accordingly, the improved hydrophilic property is reduced and returns to the original surface property.
In order to solve the foregoing problem, efforts have been continuously made to form on a metal material a hydrophilic or hydrophobic functional surface layer that can maintain a physically chemically stable state.
For example, as suggested in Japanese Laid-Open Patent Application 2001-280879, in a heat exchanger mounting a fin made of a conductive metal material on a metal tube that is a refrigerant passage, compound vapor containing titanium that is raw gas is supplied to flow in parallel to the surface of the fin of the heat exchanger in the air. Therefore, the heat exchanger coating the titanium dioxide thin film on the surface of the fin is manufactured according to a plasma CVD technology. The above patent application teaches that the heat exchanger can obtain excellent hydrophilic, antibacterial and deodorizing properties.
However, in a state where the fin is mounted on the tube to compose the heat exchanger, the titanium dioxide thin film is deposited on the fin of the heat exchanger. Thus, the titanium dioxide thin film cannot be uniformly deposited on the whole surface of the fin, which deteriorates hydrophilic and aging properties. Furthermore, productivity for direct application to the industrial production is not attained.